– Rutherford discovers a second component of radioactivity, which he calls alpha rays.

1900

– Villard discovers a third component of radioactivity, which become known as gamma rays, following the notation of Rutherford.

1905

– Einstein publishes the Special Theory of Relativity, the essential theoretical basis for understanding particle physics. From his theory he derives the equivalence of mass and energy according to the formula E=mc2.

1905

– Einstein explains the photoelectric effect as an interaction between a particle of electromagnetic radiation with an electron.

1905 to 1917

– Einstein develops the concept of the light quanta (particles of electromagnetic radiation).

– Hess discovers via balloon-borne experiments that Earth is being bombarded by penetrating radiation from above. This discovery is confirmed by Kolhörster. These radiations are later called "cosmic rays" by Millikan.

1914

– Rutherford & Andrade demonstrate the wave nature of gamma rays.

1918

– Rutherford discovers the fundamental charged particle of the atomic nucleus, which he later calls the proton.

1922

– Compton discovers that x-rays can lose energy when they scatter off electrons. This is called the Compton effect.

1924

– de Broglie hypothesizes that particles should have wave-like properties.

1925

– Pauli introduces his exclusion principle that forbids two identical half-integer-spin particles (later called Fermions) from simultaneously occupying the same quantum state. This principle is most spectacularly demonstrated by the existence of white dwarfs and neutron stars, in which degenerate electron and neutron pressure, respectively, support the interiors of these stars against gravity.

1926

– Fermi & Dirac introduce Fermi-Dirac statistics to describe the properties of particles with half-integer spin (later called Fermions), such as electrons, neutrons and protons.

– Compton defines the quantum of light as being the photon, a term previously coined by Lewis. Henceforth, x-rays and gamma rays are photons.

1927

– Clay discovers that cosmic rays are deflected by Earth's magnetic field by comparing observations at different latitudes (the "latitude effect"). He ultimately concludes that cosmic rays must be mostly charged particles.

1927

– Skobeltzyn observes tracks of high-energy charged particles in a randomly expanded cloud chamber. He concludes two years later that these charged particles must be cosmic rays.

– Bothe & Kolhörster apply the coincidence method to two Geiger-Mueller counters and discover that cosmic rays at ground level contain very-high-energy particles that can penetrate 5 cm of gold.

1930

– Rossi invents the electronic coincidence circuit to measure simultaneous pulses in multiple Geiger-Mueller counters. This technique is soon used extensively in physics experiments around the world, including in studies of cosmic rays.

1930

– Rossi predicts there should be a difference between the intensity of cosmic rays coming from the east and the west depending on the sign of their electric charge due to deflection by Earth's magnetic field (the "East-West effect").

1930

– Pauli proposes the existence of the "neutrino", a name later coined by Fermi.

1932

– Chadwick discovers the neutron. The existence of the neutron had been predicted many years earlier by Rutherford.

1932

– Anderson discovers and names the positron. The existence of the positron had been predicted by Dirac.

– Three independent experiments (by Johnson, Alvarez & Compton and Rossi) measure the East-West effect and find that the intensity of cosmic rays is greater from the west, which implies that the majority of primary cosmic rays are positively charged particles.

1934

– In the course of his East-West experiment, Rossi discovers cosmic-ray air showers, but does not study them in detail.

1935

– Yukawa predicts the existence of mesons that mediate the strong force in the atomic nucleus.

1937

– Anderson & Neddermeyer and Street & Stevenson independently announce the discovery of charged particles that become known as muons.

– Hulburt and Vegard independently propose that ionization of the upper layers of Earth's atmosphere, observed by the reflection of radio waves, is caused by ultraviolet radiation and x-rays from the sun.

– Fermi describes a process for the acceleration of non-relativistic charged particles to cosmic-ray energies via collisions with magnetic fields in the interstellar medium. This process becomes known as the "Fermi Mechanism".

1949

– Bolton, Stanley & Slee discover that the Crab Nebula is a radio source.

– Hayakawa predicts the existence of diffuse Galactic gamma-ray emission due to the decay of neutral pions that are liberated when cosmic-ray nuclei collide with interstellar gas.

1952

– Hutchinson predicts the existence of diffuse interstellar gamma-ray emission due to bremsstrahlung created by collisions of cosmic-ray electrons with interstellar matter.

1952

– Galbraith & Jelley detect Cerenkov light pulses from cosmic-ray air showers at night. That cosmic rays would contribute a small amount of light to the night sky had been predicted a few years earlier by Blackett.

1954

– Baade & Minkowski suggest that the radio source Cygnus A is two galaxies in collision.

1955

– Segre, Chamberlain et al. discover the antiproton.

1956

– A gigantic burst of neutrons is observed during a solar flare on February 23 via ground-based detectors. These are secondary neutrons produced by collisions of solar-flare protons with matter in Earth's atmosphere.

1956

– Cork et al. discover the antineutron.

1956

– Reines & Cowan announce the first definitive neutrino detection (the electron antineutrino in this case).

1956

– Hoyle & Burbidge suggest that collisions between galaxies may result in matter-antimatter annihilation, which would produce gamma rays, and could power extragalactic radio sources like Cygnus A.

1958

– Explorers 1 and 3 are launched on January 31 and March 26, respectively. Van Allen et al. discover belts of energetic charged particles in space above Earth via experiments aboard these satellites. The belts become known as the Van Allen Radiation Belts.

1958

– Peterson & Winckler detect a burst of gamma rays from a solar flare via a balloon-borne experiment. These authors are the first to make use of the term "gamma-ray burst", which will be associated with an entirely different phenomenon 15 years later.

1958

– Morrison summarizes several encouraging predictions regarding the emission of gamma rays from a variety of celestial sources. These calculations turn out to be wildly optimistic, but are key in driving the field of observational gamma-ray astronomy forward for the next several years.

– Chudakov et al. of the Lebedev Institute follow up on Cocconi's suggestion and start a search for air showers from very-high-energy gamma rays at a site in the Crimea. The experiment runs for several years, but no clear detections are made.

– The First Orbiting Solar Observatory (OSO-1) is launched on March 7. It carries several instruments, including one sensitive to high-energy gamma rays from the sun, but no such radiations are detected.

1961

– Explorer 11 is launched on April 27 carrying an instrument sensitive to gamma rays with energies above 50 MeV.

1961

– Cline sets a 95% confidence upper limit of 0.007 (cm2 s ster)-1 on the flux of celestial gamma rays at energies above 70 MeV via a balloon-borne experiment, using the first high-energy gamma-ray telescope designed for that purpose.

– Giaconni et al. discover an x-ray source that exists outside the Solar System via an experiment aboard an Aerobee rocket launched on June 19. This very strong x-ray source is named Scorpius X-1, which is eventually understood as a Low-Mass X-ray Binary (LMXB). The diffuse x-ray background is also discovered.

1962

– Schmidt makes the first redshift measurement of a quasar (3C 273). The term quasar is later coined by Chiu.

– Gell-Mann and Zweig independently put forth the quark theory of matter. The term quark is coined by Gell-Mann.

1964

– Metzger et al. present evidence for a bump in the diffuse gamma-ray background at an energy of roughly 1 MeV (the MeV bump) based on observations made via experiments carried aboard Ranger 3 and 5, both of which flew by the moon in 1962.

1965

– Kraushaar et al. announce an upper limit, based on Explorer 11 observations, of 0.0003 (cm2 s ster)-1 on the flux of celestial gamma rays with energies above 50 MeV. This limit is derived from the likely detection of only 31 celestial gamma rays. No concentration of gamma rays is noticed anywhere on the sky.

– The 3rd Orbiting Solar Observatory (OSO-3) is launched on March 8. It carries several instruments, including one sensitive to high-energy gamma rays above 50 MeV.

1967

– Vela 4a,b and the 18th Environmental Research Satellite (ERS-18) are launched on April 28. These satellites carry several experiments, including instruments sensitive to gamma rays.

1967

– Giaconni et al. announce the discovery, based on sounding rocket observations, of Cen X-3. Many years later, this source would be understood as a High-Mass X-ray Binary (HMXB) containing an accretion-powered pulsar.

1967

– Friedman & Byram detect the quasar 3C 273 and the radio galaxy M87 in x-rays via an experiment aboard and Aerobee rocket launched on May 17.

1967

– The first cosmic Gamma-Ray Burst (GRB) ever observed is detected on July 2 via the Vela 4a,b satellites. This discovery would not be made public for several years due to a military classification.

– Clark, Garmire & Kraushaar announce the detection, made via an experiment aboard OSO-3, of the Galactic Plane and Center in high-energy (above 50 MeV) gamma rays. These are the gamma rays predicted by Hayakawa. An isotropic component of high-energy gamma rays is also detected, which is thought likely to be of extragalactic origin.

1968

– Lovelace discovers the Crab Pulsar via the Arecibo Radio Telescope.

1968

– The first purpose-built atmospheric Cherenkov gamma-ray telescope is constructed in Arizona at the Mount Hopkins Observatory (later renamed the Whipple Observatory). This 10-m telescope is still in operation.

– Trombka et al. claim to confirm the detection of an excess of gamma rays of cosmic origin with an energy of roughly 1 MeV (the MeV bump) via an experiment aboard the Apollo 15 Service Module.

1973

– Kelbesadel, Strong & Olson announce the discovery of Gamma-Ray Bursts (GRBs) of cosmic origin. Their discovery paper is based on observations made from 1969 to 1972 via detectors aboard the Vela 5a,b and 6a,b satellites.

1973

– Cline et al. publish some spectra of GRBs based on data from an experiment aboard IMP-6. The observed energy spectra peak in hard x-rays and low-energy gamma rays.

1973

– Wheaton et al. announce the detection, made via x-ray telescopes on OSO-7, of x-ray emission down to energies below 10 keV from a GRB.

– Celestial Observation Satellite B (COS-B) is launched on August 9. It carries a digitized spark chamber sensitive to high-energy gamma rays, which operates successfully for more than six years.

1975

– Kniffen et al. announce the detection, made via SAS-2 observations, of an excess of high-energy gamma-ray radiation from the Galactic Anticenter region that cannot be tied to any known source. Bignami et al. subsequently apply the name "Geminga" to this mysterious object.

– Helios 2 is launched on January 15. Included in its instrumentation is a tiny experiment by Cline et al. that is the first purpose-built GRB detector. The spacecraft goes into orbit around the sun. The Helios 2 experiment along with instruments in orbit near Earth initiate the first Inter-Planetary Network (IPN) of GRB detectors. This modest network can localize a GRB to a narrow swath on the sky.

1977

– The 1st High Energy Astrophysical Observatory (HEAO 1) is launched on August 12. It carries several x-ray and gamma-ray experiments.

1977

– Leventhal et al. conclusively demonstrate via a balloon-borne experiment that the emission from the Galactic Center is due to 511-keV positron annihilation.

1978

– Swanenburg et al. discover that quasar 3C 273 is a source of high-energy gamma rays based on COS-B observations.

1978

– The Pioneer Venus Orbiter (PVO) is launched on May 20. It carries several instruments, including a GRB detector. It goes into orbit around Venus on December 4. The GRB detector functions until 1992.

1978

– The 3rd International Sun Earth Explorer (ISEE-3) is launched on August 12. It carries several instruments, including detectors designed to observe solar flares and GRBs. The spacecraft is renamed the International Cometary Explorer (ICE) in 1982.

1978

– Venera 11 and 12 are launched on September 9 and 14, respectively. These spacecraft carry many experiments, including Konus and SIGNE 2 GRB detectors. The flight platforms fly by Venus on December 25 and 21, respectively. These, with Helios-2 and PVO, complete the first IPN, which localizes many GRBs to arc-minute-sized regions of "blank" sky.

1978

– Kniffen et al. announce the definitive measurement, made via a balloon-borne digitized spark chamber, of gamma-ray emission from the Galactic Center region in the 15 to 100 MeV range.

1978

– Prognoz 7 is launched on October 30. It carries several instruments, including SIGNE 2 GRB detectors.

1979

– An enormously intense burst of low-energy gamma rays is observed on March 5 (the March 5 event) via detectors aboard many satellites. Mazets et al. detect an 8-s periodicity in the lightcurve of the event via the Konus detectors aboard Venera 11 and 12, and they also notice additional events from the same source. Evans et al. use the IPN to tie the source to the SuperNova Remnant (SNR) N49 in the Large Magellanic Cloud (LMC). Eventually, the March 5 event source is understood as being the first member of a new family of sources that become known as Soft Gamma Repeaters (SGRs). These are distinct from the classical GRB sources. This particular object receives the designation SGR 0526-66.

1979

– Mazets et al. announce the discovery, based on Venera 11 and 12 observations, of a second SGR, which becomes known as SGR 1900+14.

1979

– The 3rd High Energy Astrophysical Observatory (HEAO 3) is launched on September 20. It carries several experiments, including a high resolution gamma-ray spectrometer.

1980

– The Solar Maximum Mission (SMM) is launched on February 14. One of the instruments it carries is called the Gamma-Ray Spectrometer (GRS). Another is the Hard X-Ray Burst Spectrometer (HXRBS) that is sensitive to photons up to energies of 500 keV.

1980

– Hudson et al. announce the detection, made via an experiment aboard HEAO 1, of the 2.223 and 4.43 MeV lines during a large solar flare in July 1978. This was the first confirmation of the solar 2.223 MeV neutron-capture line that was initially observed in 1972.

1980

– Chupp et al. detect neutrons from the sun during a solar flare in June via the GRS aboard SMM. This is the first such detection, confirming a prediction made three decades earlier by Biermann et al.

– Swanenburg et al. release the Second COS-B Catalog of high-energy gamma-ray sources. The majority of these sources are unidentified.

1981

– Venera 13 and 14 are launched on October 30 and November 4, respectively. Each spacecraft carries several instruments, including Konus GRB detectors. The flight platforms fly by Venus on March 1 and 4 of 1982, respectively.

1982

– Mayer-Hasselwander et al. publish a detailed map of the Galactic Plane in high-energy gamma rays based on COS-B observations.

1982

– Prince et al. announce, based on HEAO 3 gamma-ray spectrometer observations from November 1979, the first high-spectral-resolution measurement of the 2.223 MeV neutron-capture line during a solar flare.

1983

– Samorski & Stamm publish evidence for PeV gamma rays from the Galactic x-ray binary source Cygnus X-3, as detected by the Kiel air-shower array. This detection is apparently subsequently confirmed by observations made by other air-shower arrays and atmospheric Cherenkov telescopes. However, the statistical significance of all the results is weak. In the end, Cygnus X-3 and similar object Hercules X-1 are not confirmed as emitters of TeV or PeV gamma rays, but the huge excitement from the putative detections greatly increases the interest in very-high-energy gamma-ray astronomy.

– Mahoney et al. announce the discovery, based on HEAO 3 gamma-ray spectrometer observations, of a gamma-ray emission line at 1.81 MeV from the Galactic Plane. This radiation is due to the decay of 26Al, a radioactive isotope of aluminum that is produced in supernovae.

1985

– Share et al. announce the detection, made via the GRS aboard SMM, of the gamma-ray emission line from the Galactic Center at 1.81 MeV due to the decay of 26Al.

1985

– Forrest et al. announce the detection, made via the GRS aboard SMM, of meson-decay gamma rays in a solar flare in June 1982.

1986

– Laros et al. announce the discovery, based on IPN observations, of a third SGR, which becomes known as SGR 1806-20.

– Weekes et al. publish the first firm detection of TeV gamma rays from an astrophysical source. This detection of the Crab Nebula was made via the Whipple Observatory 10-m reflector using the atmospheric Cherenkov imaging technique.

1989

– Granat is launched on December 1. It carries several instruments that can detect x-rays and gamma rays, including GRB detectors, and the SIGMA coded-aperture telescope that can image the sky in low-energy gamma rays.

1990

– Leising & Share publish a gamma-ray lightcurve for SN1987A based on SMM GRS observations. The lightcurve is powered by the radioactive decay of 56Co.

1990

– The "Gamma" spacecraft is launched on July 11. It carries the Gamma-1 telescope that is sensitive to high-energy gamma rays. Unfortunately, the high-voltage power supply for the spark chamber in this instrument fails shortly after launch, greatly reducing its angular resolution.

1990

– Ulysses is launched on October 6. It carries several instruments, including a GRB experiment. The spacecraft's 5-AU solar-polar orbit carries it well out of the plane of the ecliptic, which provides excellent additional baseline for the IPN for the next 18 years.

1990

– ROSAT (Röntgensatellit) is launched on June 1. This observatory is sensitive to extreme ultraviolet photons and x-rays. It would go on to observe well over a hundred thousand x-ray sources, which would prove to be a useful asset for identifying gamma-ray sources.

– Akimov et al. detect gamma rays extending to 1 GeV via the Gamma-1 telescope on the Gamma spacecraft during solar flares on March 26 and June 15.

1991

– Several strong solar flares in June are observed by all four instruments aboard CGRO. OSSE detects several gamma-ray emission lines from a solar flare on June 4. EGRET detects high-energy gamma-ray emission from a solar flare on June 11. COMPTEL detects neutrons from a solar flare on June 15, and these data are used to create the first "image" of a star (i.e., the sun) in particles other than photons.

1991

– The University of Utah's "Fly's Eye I" experiment detects a 3.2 x 1020 eV cosmic ray on October 15, the most energetic particle ever detected.

1992

– Meegan et al. announce two discoveries based on BATSE observations: the GRBs are distributed isotropically on the sky, and there are fewer faint bursts than expected if the bursts sources are distributed uniformly throughout space. These results grow stronger as the observations accumulate, suggesting that the GRB sources are located at cosmological distances. The final BATSE Catalog would ultimately contain 2704 GRBs.

1992

– Hartman et al. announce the detection, made via EGRET observations, of the quasar 3C 279 in high-energy gamma rays. This represents the discovery of "blazars" as being a class of powerful and variable sources.

1992

– Punch et al. announce the detection, made via Whipple Observatory observations, of TeV photons from the blazar Markarian 421. This is the first extragalactic TeV source to be discovered.

1992

– Sreekumar et al. announce the detection, made via EGRET observations, of high-energy gamma rays from the LMC, which is the first detection in gamma rays of a normal galaxy beyond the Milky Way. It is quite certain that these gamma rays result from the collisions of cosmic rays with gas within that galaxy, and the conclusion is reached that the cosmic-ray density in the LMC is the same as in the Milky Way.

1992

– Halpern & Holt announce the discovery, based on ROSAT observations, of soft x-ray pulsations from Geminga. Bertsch et al. announce the discovery, based on EGRET observations, of Geminga's high-energy gamma-ray pulsations. Geminga is finally identified; it is a rotation-powered pulsar.

1992

– Duncan & Thompson and (independently) Paczynski propose that the March 5 event source (SGR 0526-66) is a highly-magnetized (~5 x 1015 G) neutron star. They suggest that a "starquake" in the crust of such an object can result in a disturbance in the magnetic field that can cause a strong gamma-ray outburst. Such a neutron star is called a "magnetar".

1992

– Mirabel & Rodriguez announce the discovery, based on Very Large Array (VLA) radio observations, that the Galactic x-ray and gamma-ray source 1E140.7-2942 has a pair of radio jets. It is dubbed a "microquasar", and is the first known example. This source is also called "X-Ray Nova Muscae" and the "Galactic Center Annihilator". It is an LMXB that contains a black hole. A year earlier, variable gamma-ray emission at 511 keV from this source was discovered by Bouchet et al. via the SIGMA instrument on GRANAT.

1992

– Kurfess et al. announce, based on OSSE observations, the first direct measurement of the mass of 57Co produced in SN1987A. The ratio of 57Ni/56Ni is estimated to be slightly larger than, but consistent with, the solar ratio of 57Fe/56Fe. This is a great improvement over earlier indirect estimates, which yielded much higher values for the ratio.

1993

– The Advanced Satellite for Cosmology and Astrophysics (ASCA) is launched on February 23. This observatory is sensitive to x-rays. It would become a very successful mission, which includes helping to identify several gamma-ray sources.

1993

– Kanbach et al. announce the detection, made via EGRET observations, of gamma rays, with energies up to 1 GeV, for eight hours after a solar flare on 1991 June 11. These gamma rays are due to meson decay and electron bremsstrahlung.

1993

– Kouveliotou et al. announce the discovery, based on BATSE observations, that the so-called "short" and "long" GRBs differ spectroscopically, in that the short bursts tend to be harder than the long bursts. The dividing line between the groups is found to be at a burst duration of 2 seconds.

1994

– Hurley et al. detect on February 17 high-energy gamma-ray emission during a GRB via EGRET observations. This high-energy emission continues long after the low-energy gamma-ray emission from the burst ceases, and includes an 18-GeV photon that arrives 90 minutes after the burst began. These high-energy photons would later be understood as being a component of the GRB afterglow.

1994

– Observations from the SIGMA instrument on GRANAT are used to discover the source GRS 1915+105 on August 15, which becomes known as "Old Faithful", due to its semi-regular hard-x-ray/soft-gamma-ray outbursts that occur every 45 to 90 minutes. Mirabel & Rodriguez announce in September that this source is the first microquasar in our Galaxy known to exhibit superluminal motion.

1994

– Iyudin et al. announce the detection, made via COMPTEL observations, of the radioactive decay of 44Ti at 1.16 MeV in the Cas A SNR.

1994

– The WIND spacecraft is launched on November 1. Included amongst its instruments are the Transient Gamma-Ray Spectrometer (TGRS) and a KONUS GRB detector.

1995

– Paczynksi and Lamb debate each other in Washington DC on April 22 regarding the distance scale to the GRBs. After the debate, the audience is split or undecided on whether the bursts lie at cosmological distances or within the halo of our Galaxy.

1995

– Diehl et al. release the first map at 1.809 MeV of the entire Galactic Plane, based on COMPTEL observations, and estimate the total amount of radioactive 26Al in the Galaxy to be less than or equal to one solar mass.

1995

– Naya, Tueller et al. announce, based on GRIS balloon-borne observations, the first firm measurement of the width of the 26Al line at 1.809 MeV line in the Galactic Center Region.

1995

– The Rossi X-ray Timing Explorer (RXTE) is launched on December 30. This observatory is sensitive to x-rays and soft gamma-rays. It would become a very successful mission, which includes helping to identify many gamma-ray sources.

1996

– The BeppoSAX (Satellite per Astronomia X) observatory is launched on April 30. This observatory is sensitive to x-rays and soft gamma-rays. It would become a very successful mission, which includes localizing many GRB afterglows on the sky with arc-minute accuracy.

1996

– Prompt x-ray emission from a GRB is imaged via a BeppoSAX Wide Field Camera (WFC) on July 20, and a coarse localization is obtained. A variety of follow-up observations are carried out, but these are done much too late to detect an afterglow, and, hence, no fine localization is made.

1997

– Several x-ray counterparts of GRBs are finely localized via BeppoSAX WFC and Narrow Field Instrument (NFI) observations, the first being on February 28. These are ultimately used to help identify optical and radio counterparts to GRBs. The May 8 event is especially important, because it is the first to result in a measured redshift (= 0.835), and the decay of the radio afterglow reveals behavior indicative of a relativistic jet. This and subsequent evidence leads to the conclusion that so-called "long" GRBs are enormous explosions that occur in star forming regions of galaxies at cosmological distances.

1997

– Johnson et al. announce the detection, made via OSSE observations, of gamma rays with energies up to 300 keV from the Seyfert galaxy NGC 4151.

1997

– Remarkable TeV gamma-ray flares are detected from the blazar Markarian 501, and are followed around the clock with several atmospheric Cherenkov telescopes: Whipple (in Arizona), HEGRA (High Energy Gamma Ray Astronomy array, on La Palma), CAT (Cherenkov Array at Themis, in France), and TAP (Telescope Array Prototype, in Utah).

1998

– BeppoSAX localizes a GRB on April 25 that is circumstantially tied to an underluminous and nearby (redshift = 0.0085) supernova known as SN1998bw.

1998

– Kouveliotou et al. announce the discovery, based on RXTE and ASCA observations, of x-ray pulsations from SGR 1806-20 that suggest the underlying object is a neutron star with a dipole magnetic field strength equal to that of a magnetar. A similar conclusion is reached for SGR 1900+14 several months later. Henceforth, SGRs are generally regarded as magnetars.

1998

– Kouveliotou et al. announce the discovery, based on BATSE observations, of a fourth SGR, which becomes known as SGR 1627-41.

1998

– A giant outburst from SGR 1900+14 is widely observed on August 27, which results in the shutdown of several spacecraft, and affects radio communications on Earth due to the increased ionization of the outer atmosphere.

1998

– Iyudin et al. announce the discovery, based on COMPTEL observations, of 44Ti emission at 1.16 MeV from an SNR in the Vela Region.

1999

– An extremely luminous GRB is observed on January 23. The 2nd Robotic Optical Transient Search Experiment (ROTSE-II) detects prompt optical emission bright enough to have been visible by an Earth-bound observer with binoculars, which is remarkable given the great distance (redshift = 1.60).

1999

– Hartman et al. release the Third EGRET Catalog, which includes 271 high-energy gamma-ray sources above 100 MeV. The majority of the sources, 170, are unidentified. The identified sources include 93 blazars, 5 pulsars, a radio galaxy (Cen A), a normal galaxy (LMC), and the sun.

– The Milagro experiment in New Mexico, based on the water Cerenkov technique, becomes fully operational in January, and runs around the clock. Eventually this instrument is used to carry out a full survey of the northern sky for gamma rays at TeV energies. Several new sources, including extended ones, are discovered in the Galactic Plane, along with diffuse Galactic gamma-ray emission.

2000

– Schoenfelder et al. release the First COMPTEL Source Catalog. It covers the energy range from 0.75 to 30 MeV. The catalog contains 32 steady sources, 31 GRBs and 21 solar flares. The steady sources include spin-down pulsars, stellar-mass black holes, SNRs, interstellar clouds, and Active Galactic Nuclei (AGNs). Line detections include the 26Al line at 1.809 MeV, the 44Ti line at 1.157 MeV, the 56Co lines at 0.847 MeV & 1.238 MeV, and the neutron-capture line at 2.223 MeV.

2000

– CGRO disintegrates in Earth's atmosphere on June 4 following a controlled re-entry. The decision to deliberately re-enter the spacecraft came after the failure of one of its gyroscopes on 1999 December 19.

2000

– HETE-2 (High Energy Transient Explorer) x-ray observatory is launched on October 9, which is primarily designed to study GRBs.

2002

– RHESSI (Ramaty High Energy Solar Spectroscopic Imager) solar observatory is launched on February 5. Soon after, gamma rays from solar flares are imaged for the first time.

– The H.E.S.S. (High Energy Stereoscopic System) array of four atmospheric Cherenkov telescopes is inaugurated in Namibia in September.

2004

– Swift GRB Explorer is launched on November 20.

2004

– A giant outburst from SGR 1806-20 is observed on December 27 by Swift, RHESSI and INTEGRAL.

2005

– A very powerful gamma-ray-line solar flare is observed via RHESSI on January 20. This flare exhibits very strong evidence for meson-decay gamma rays.

2005

– Harris et al. announce the marginal detection, made via the cooled germanium spectrometer (SPI) aboard INTEGRAL, of gamma-ray emission from the decay of 60Fe in the Galactic Plane at 1.173 and 1.333 MeV. The 60Fe/26Al ratio is estimated. This result is firmed up two years later by Wang et al.

2005

– Swift and HETE-2 observations in May and July localize x-ray counterparts for the so-called "short" GRBs. It is found that these short bursts are associated with galaxies, but not with star formation regions within the galaxies. This circumstantial evidence suggests these events may be due to mergers of pairs of compact objects (e.g., two neutron stars, or a neutron star and a black hole).

2005

– H.E.S.S. is used to discover many new sources of TeV gamma rays, including SNRs, pulsar wind nebulae, the Galactic Center, a binary pulsar, an x-ray binary, and numerous new blazars.

2006

– MAGIC-I is used to discover TeV gamma rays from black hole candidate Cygnus X-1 and quasar 3C 279. The latter is the first quasar to be detected at TeV energies.

2006

– The Swift satellite localizes two "long" GRBs in the late Spring that are subsequently quite thoroughly studied but are clearly not associated with supernovae.

2007

– AGILE (Astro-rivelatore Gamma a Immagini LEggero) is launched on April 23. It carries an instrument that is sensitive to high-energy gamma rays.

– Weidenspointner et al. announce the discovery, based on INTEGRAL SPI observations, that the 511-keV annihilation-line radiation from the Galactic Center is lopsided. The distribution of 511-keV intensity correlates with the locations of LMXBs. The LMXBs are suggested to be the likely source of at least some of these gamma rays.

2008

– The apparently brightest GRB ever is detected on March 19 via the Swift satellite and several ground-based instruments. The optical emission was bright enough to have been briefly visible to the naked eye, in spite of the large distance (redshift = 0.937).

2008

– VERITAS is used to detect TeV photons from the intermediate BL Lac object W Comae.

2008

– MAGIC-I is used to detect the Crab Pulsar. This is the first detection of a pulsar by a ground-based gamma-ray telescope.

2008

– The Fermi Gamma-ray Space Telescope (formerly known as GLAST, the Gamma-ray Large Area Space Telescope) is launched on June 11. It carries an instrument that is exceptionally sensitive to high-energy gamma rays, as well as a GRB monitor.

2008

– A young, radio-quiet pulsar is discovered in SNR "CTA 1" via Fermi/GLAST observations. Several of the unidentified EGRET sources in star-forming regions and near SNRs turn out to be such pulsars.

2008

– The most energetic GRB ever detected is observed on September 16 via the Swift and Fermi satellites. It is the first GRB detected by the Fermi LAT (Large Area Telescope). The burst is twice as energetic as GRB990123, the previous record holder.

2009

– The most distant GRB ever observed is detected on April 23 via the Swift satellite. Follow-up ground-based observations measure the redshift to be 8.2, which translates into a distance of more than 13 billion light years. This GRB is also the most distant object ever detected by humankind, except for the CMB.